Is voluntary fasting sufficient in place of obligatory fasting?
Machine translationRead the Arabic original
Question : What are they saying about the prayer?
Answer : No, it cannot be a substitute for an obligatory act; it is not valid and does not serve as a substitute for an obligation, because the Prophet (peace and blessings be upon him) said:Sahih al-Bukhari 1 Therefore, since he did not intend the obligation, it is not valid. As I told you, the safest course is that one should not perform voluntary acts until one has fulfilled the obligation.
Answer : No, it cannot be a substitute for an obligatory act; it is not valid and does not serve as a substitute for an obligation, because the Prophet (peace and blessings be upon him) said:
The reward of deeds depends upon the intentions and every person will get the reward according to what he has intended.
إنما الأعمال بالنيات وإنما لكل امرئ ما نوى
Source
قناة «فضيلة الشيخ محمد بن صالح العثيمين»، 328- هل يُجزئ صيام النافلة عن صيام الفريضة؟ II فوائد من الدروس الفقهية من المحاضرات الجامعية — youtube.com
The Arabic text is a machine transcription of the audio recording.